Display Device and Manufacturing Method of the Same

ABSTRACT

A display device free of contact resistance between a drain electrode (or a source electrode) and a pixel electrode. The display device includes a gate electrode, a gate insulating layer covering the gate electrode, a semiconductor layer formed over the gate insulating layer, and a source electrode and a drain electrode separated from each other and in partial-contact with and over the semiconductor layer, and one of the source electrode and the drain electrode also serves as a pixel electrode, the other of the source electrode and the drain electrode also serves as a signal line, and a low resistant conductive layer is preferably formed over the other of the source electrode and the drain electrode. The low resistant conductive layer can be formed by an electroplating method or the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device and a method formanufacturing the display device. Further, the present invention relatesto a production system of the display device.

Note that the term “display device” in this specification includes pixeltransistors included in a display device and an active matrix substrateprovided with a plurality of pixel transistors, as well as the displaydevice itself.

2. Description of the Related Art

In recent years, display devices such as liquid crystal display devicesand EL display devices are spreading rapidly. Such display devices areclassified roughly into passive matrix display devices and active matrixdisplay devices. In an active matrix display device, an active matrixsubstrate in which a plurality of switching elements are provided inmatrix is used. As a switching element, a transistor is usually used forexample.

A general active matrix substrate is provided with a pixel transistorincluding: a thin film transistor which includes a scan line alsoserving as a gate electrode, a signal line also serving as a sourceelectrode (or a drain electrode), a semiconductor layer, a drainelectrode (or a source electrode); and a protective insulating layercovering the thin film transistor; and a pixel electrode connected tothe drain electrode (or the source electrode) in an opening portionprovided in the protective insulating layer.

In such an active matrix substrate, a contact resistance of a portionwhere the drain electrode (or the source electrode) is in contact withthe pixel electrode in the opening portion is one of hindrances toimprovement of display characteristics (for example, see paragraph[0005] in Patent Document 1).

In addition, in manufacturing an active matrix substrate, it is knownthat an increase in the number of masks used leads to an increase in themanufacturing cost. Thus, it is well-known that many approaches toreduce the number of masks used have been made, without needing tomention a reference.

Note that, in this specification, the term “mask” includes an etchingmask, a photomask, and a protective mask. Note that the “photomask”means a mask layer used in light exposure in a photolithography method,the “etching mask” means a mask layer formed to prevent a film formedunder the etching mask from being etched, and the “protective mask”means a mask layer formed to prevent a film formed over the protectivemask from being in contact with a layer formed under the protectivemask. Note that the “etching mask” and the “protective mask” can each beformed of a resist material, and thus can be referred to as a resistmask in some cases.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    H7-120790-   [Patent Document 2] Japanese Published Patent Application No.    2002-318555

SUMMARY OF THE INVENTION

It is an object of one embodiment of the present invention to provide adisplay device free of contact resistance between a drain electrode (ora source electrode) and a pixel electrode.

It is another object of one embodiment of the present invention toprovide a display device free of contact resistance between a drainelectrode (or a source electrode) and a pixel electrode, withoutincreasing a wiring resistance.

Further, it is another object of one embodiment of the present inventionto provide a manufacturing method of a display device by which thenumber of masks can be smaller than that by a conventional method.

One embodiment of the present invention is a display device in which adrain electrode (or a source electrode) is formed in the same layer as apixel electrode in an active matrix substrate.

One embodiment of the present invention is a display device including agate electrode; a gate insulating layer formed to cover the gateelectrode; a semiconductor layer formed over the gate insulating layer;and a source electrode and a drain electrode which are separated fromeach other and in contact with the semiconductor layer, wherein one ofthe source electrode and the drain electrode also serves as a pixelelectrode.

Another embodiment of the present invention is a display deviceincluding a gate electrode; a gate insulating layer formed to cover thegate electrode; a semiconductor layer formed over the gate insulatinglayer; and a source electrode and a drain electrode separated from eachother and in contact with the semiconductor layer, wherein one of thesource electrode and the drain electrode also serves as a pixelelectrode; the other of the source electrode and the drain electrodeserves as a signal line; and a low resistant conductive layer is formedover the other of the source electrode and the drain electrode. Notethat in this specification, the low resistant conductive layer may havea lower resistance than at least the electrode also serving as a signalline and the electrode also serving as a pixel electrode.

Another embodiment of the present invention is a method formanufacturing a display device, comprising the steps of forming a firstconductive film over a substrate; forming a first etching mask over thefirst conductive film; processing the first conductive film with use ofthe first etching mask, whereby a gate electrode is formed; removing thefirst etching mask; forming a gate insulating layer over the gateelectrode; forming a semiconductor film over the gate insulating layer;forming a second etching mask over the semiconductor film; processingthe semiconductor film with use of the second etching mask, whereby asemiconductor layer is formed; removing the second etching mask; forminga transparent conductive film over the gate insulating layer to coverthe semiconductor layer; forming a third etching mask over thetransparent conductive film; processing the transparent conductive filmwith use of the third etching mask, whereby an electrode also serving asa signal line and an electrode also serving as a pixel electrode areformed; and removing the third etching mask. Note that after that,preferably, etching of an upper portion of the semiconductor layer isconducted to remove residue or the like left between the electrode alsoserving as a signal line and the electrode also serving as a pixelelectrode.

Another embodiment of the present invention is a method formanufacturing a display device, comprising the steps of forming a firstconductive film over a substrate; forming a first etching mask over thefirst conductive film; processing the first conductive film with use ofthe first etching mask, whereby a gate electrode is formed; removing thefirst etching mask; forming a gate insulating layer over the gateelectrode; forming a semiconductor film over the gate insulating layer;forming a second etching mask over the semiconductor film; processingthe semiconductor film with use of the second etching mask, whereby asemiconductor layer is formed; removing the second etching mask; forminga transparent conductive film over the gate insulating layer to coverthe semiconductor layer; forming a third etching mask over thetransparent conductive film; processing the transparent conductive filmwith use of the third etching mask, whereby an electrode also serving asa signal line and an electrode also serving as a pixel electrode areformed; removing the third etching mask; and forming a low resistantconductive layer only over the electrode also serving as the signalline. Note that after that, preferably etching of an upper portion ofthe semiconductor layer is conducted to remove residue or the like orthe like left between the electrode also serving as a signal line andthe electrode also serving as a pixel electrode.

In the manufacturing method of the above-described display deviceaccording to one embodiment of the present invention, the low resistantconductive layer may be a metal layer formed by an electroplatingmethod. When the low resistant conductive layer is formed by anelectroplating method, the substrate where elements up to and includingthe electrode also serving as a signal line and the electrode alsoserving as a pixel electrode are formed, and an anode formed of a metalto be used for plating or an insoluble metal are immersed in anelectrolyte solution containing ions of the metal to be used forplating, so that a potential difference is generated between the anodeand the electrode also serving as a signal line and thus cations arereduced on the surface of the electrode also serving as a signal line,and thereby a metal layer is formed. After that, preferably etching ofan upper portion of the semiconductor layer is conducted to removeresidue or the like left between the electrode also serving as a signalline and the electrode also serving as a pixel electrode. Further, themetal to be used for plating may be a metal having a low resistance anda low ionization tendency.

The technique by which a signal line is treated by an electroplatingmethod is disclosed in e.g., Patent Document 2.

In addition, in the manufacturing method of the above-described displaydevice according to one embodiment of the present invention, in order togenerate a potential difference between the anode and the electrode alsoserving as a signal line when the metal layer is formed by anelectroplating method, the potential input to the signal line may besupplied from an arm transporting the substrate on the surface incontact with a signal line side input terminal over the substrate. Inother words, the arm transporting the substrate has a conductive layerin its portion in contact with the signal line side input terminal.Accordingly, one embodiment of the present invention is a productionsystem of a display device, wherein the substrate where elements up toand including the electrode also serving as a signal line and theelectrode also serving as a pixel electrode are formed according to themanufacturing method of the above-described display device according toone embodiment of the present invention is transported by the arm havinga conductive layer in contact with the signal line side input terminalover the substrate, the substrate and an anode formed of a metal to beused for plating or an insoluble metal are immersed in an electrolytesolution containing ions of the metal to be used for plating with thesubstrate held by the arm so that a potential difference is generatedbetween the anode and the conductive layer of the arm, and thereby ametal layer is formed on the surface of the electrode also serving as asignal line.

In the above-described display device according to one embodiment of thepresent invention, a sidewall insulating layer is preferably formed onthe side surfaces of the electrode also serving as a signal line and theelectrode also serving as a pixel electrode. In other words, in themanufacturing method of the display device according to one embodimentof the present invention, after removing the third etching mask, aninsulating film (hereinafter, referred to as a sidewall insulating film)for formation of the sidewall insulating layer is formed over the gateinsulating layer, the semiconductor layer, the electrode also serving asa signal line and the electrode also serving as a pixel electrode, andetch-back treatment is conducted to expose the electrode also serving asa signal line and the electrode also serving as a pixel electrode sothat the sidewall insulating layer is formed, then, a low resistantconductive layer is preferably formed over the electrode also serving asa signal line by an electroplating method. The formation of the sidewallinsulating layer in contact with the side surfaces of the electrode alsoserving as a signal line and the electrode also serving as a pixelelectrode makes it possible to set the distance (channel length L)between the electrode also serving as a signal line and the electrodealso serving as a pixel electrode at the time of processing thetransparent conductive film, irrespective of the formation method of thelow resistant conductive layer. Accordingly, the channel length can beprevented from varying from point to point within the substrate, andthus display unevenness (mura) of the display device can be prevented.After that, preferably, etching of an upper portion of the semiconductorlayer is conducted to remove residue or the like left between theelectrode also serving as a signal line and the electrode also servingas a pixel electrode.

In the manufacturing method of the display device according to oneembodiment of the present invention, when the sidewall insulating layeris formed, the electrode also serving as a signal line and the electrodealso serving as a pixel electrode are easily subjected to plasmadamages. Therefore, after the electrode also serving as a signal lineand the electrode also serving as a pixel electrode are formed,preferably, the surfaces of the electrodes are slightly etched, orsubjected to wet etching or washing so that the plasma damages areremoved.

Alternatively, in the manufacturing method of the display deviceaccording to one embodiment of the present invention, the low resistantconductive layer may be formed by a method other than an electroplatingmethod. For example, the following method may be employed: after thethird etching mask is removed, a protective mask is farmed by a methodthat does not use a photomask such as an inkjet method in a regionexcluding the region where the low resistant conductive layer will beformed, a low resistant conductive film is formed over the electrodealso serving as a signal line and the protective mask, and then theprotective mask is lifted off so that the low resistant conductive layeris formed. After the protective mask is removed, preferably, etching ofan upper portion of the semiconductor layer is conducted to removeresidue or the like left between the electrode also serving as a signalline and the electrode also serving as a pixel electrode.

Alternatively, in the manufacturing method of the display deviceaccording to one embodiment of the present invention, the low resistantconductive layer may be formed by a droplet-discharge method instead ofa lift-off process. For example, the following method may be employed:after the third etching mask is removed, the low resistant conductivefilm is formed, a fourth etching mask is formed by a droplet-dischargemethod in the region where the low resistant conductive layer will beformed over the low resistant conductive film, the low resistantconductive film is processed with use of the fourth etching mask, sothat the low resistant conductive layer is formed, and then the fourthetching mask is removed. After the fourth etching mask is removed,preferably, etching of an upper portion of the semiconductor layer isconducted to remove residue or the like left between the electrode alsoserving as a signal line and the electrode also serving as a pixelelectrode.

The display device described above or a display device manufactured bythe manufacturing method of the display device can be applied to displayportions of electronic devices.

Note that in this specification, a “film” refers to a film which isformed over the entire surface of an object by a CVD method (including aplasma CVD method and the like), a sputtering method, or the like. Onthe other hand, a “layer” refers to a layer which is formed byprocessing a “film” or a layer which is formed over the entire surfaceof an object and does not require to be subjected to processing.However, the words “film” and “layer” are not necessarily distinguishedin some cases.

Although an ohmic contact layer between a semiconductor layer and sourceand drain electrodes is not described in this specification, an ohmiccontact layer is preferably provided between a semiconductor layer andsource and drain electrodes. The ohmic contact layer can be formed insuch a way that an element imparting one conductivity type (phosphorus,arsenic, or the like for an n-channel transistor, and boron or the likefor a p-channel transistor) is added to a semiconductor layer.

According to one embodiment of the present invention, since a drainelectrode (or a source electrode) also serves as a pixel electrode, adisplay device free of contact resistance between the drain electrode(or the source electrode) and the pixel electrode can be provided.

Further, in accordance with a preferable embodiment of the presentinvention, a low resistant conductive layer is provided over anelectrode also serving as a source electrode (or drain electrode) andalso serving as a signal line, and thereby a display device free ofcontact resistance between the drain electrode (or the source electrode)and the pixel electrode can be provided, without increasing a wiringresistance.

Furthermore, according to a manufacturing method of a display device asone embodiment of the present invention, the number of masks can besmaller than that in a conventional method.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1D illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIGS. 2A to 2D illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIG. 3 illustrates a pixel transistor according to one embodiment of thepresent invention;

FIGS. 4A and 4B illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIG. 5 illustrates a pixel transistor according to one embodiment of thepresent invention;

FIGS. 6A and 6B illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIGS. 7A and 7B illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIGS. 8A and 8B illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIGS. 9A and 9B illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIGS. 10A and 10B illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIGS. 11A to 11D illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIGS. 12A to 12D illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIGS. 13A to 13D illustrate a fabrication method of a pixel transistoraccording to one embodiment of the present invention;

FIGS. 14A and 14B illustrate a production system of a display deviceaccording to one embodiment of the present invention;

FIG. 15 illustrates a semiconductor device to which a display device ofone embodiment of the present invention is applied;

FIGS. 16A and 16B each illustrate a semiconductor device to which adisplay device of one embodiment of the present invention is applied;and

FIG. 17 illustrates a semiconductor device to which a display device ofone embodiment of the present invention is applied.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed without departing from the scope and spirit of thepresent invention. Accordingly, the present invention should not beconstrued as being limited to the description of the embodiments below.

Embodiment 1

In this embodiment, a pixel transistor formed in a display device,according to one embodiment of the present invention, and a fabricationmethod thereof will be described.

First, a fabrication method of a pixel transistor in this embodimentwill be described. One feature of the fabrication method of a pixeltransistor in this embodiment lies in that a first conductive film isformed over a substrate, a first etching mask is formed over the firstconductive film, the first conductive film is processed with use of thefirst etching mask so that a gate electrode is formed, the first etchingmask is removed, a gate insulating layer is formed to cover the gateelectrode, a semiconductor film is formed over the gate insulatinglayer, a second etching mask is formed over the semiconductor film, thesemiconductor film is processed with use of the second etching mask sothat a semiconductor layer is formed, the second etching mask isremoved, a transparent conductive film is formed to cover thesemiconductor layer over the gate insulating layer, a third etching maskis funned over the transparent conductive film, the transparentconductive film is processed with use of the third etching mask so thatan electrode also serving as a signal line and an electrode also servingas a pixel electrode are formed, and the third etching mask is removed.

First, a first conductive film 102 is formed over a substrate 100, and afirst etching mask 104 is formed over the first conductive film 102(FIG. 1A).

Non-limiting examples of the substrate 100 includes a glass substrate, aquartz substrate, a silicon substrate, a stainless steel substrate, anda plastic substrate.

As the first conductive film 102, a conductive film (for example, ametal film, a semiconductor film doped with an impurity elementimparting one conductivity type, or the like) may be formed by forexample a sputtering method or a CVD method (including a plasma CVDmethod, a thermal CVD method, and the like). Alternatively, the firstconductive film 102 may be formed by an inkjet method. Note that thefirst conductive film 102 may be formed to have either a single-layerstructure or a stacked-layer structure including a plurality of layers.For example, the first conductive film 102 may be formed with athree-layer structure in which an Al layer is sandwiched between Tilayers or Mo layers.

As the first etching mask 104, for example, a resist mask may be funned;however, there is no particular limitation on the first etching mask104, and any material that can be used as a mask in an etching step canbe used.

Next, with the use of the first etching mask 104, the first conductivefilm 102 is processed so that the gate electrode 106 is formed (FIG.1A).

Then, the first etching mask 104 is removed (FIG. 1B).

Then, a gate insulating layer 108 is formed to cover the gate electrode106, a semiconductor film 110 is fowled over the gate insulating layer108, and a second etching mask 112 is formed over the semiconductor film110 (FIG. 1C).

As the gate insulating layer 108, for example, a film of an insulatingmaterial (for example, silicon nitride, silicon nitride oxide, siliconoxynitride, silicon oxide, or the like) may be formed by a method suchas a sputtering method or a CVD method (including a plasma CVD method, athermal CVD method, and the like). Note that the gate insulating layer108 may be formed to have either a single-layer structure or astacked-layer structure including a plurality of layers.

Note that “silicon nitride oxide” contains more nitrogen than oxygen andin the case where measurements are performed using Rutherfordbackscattering spectrometry (RBS) and hydrogen forward scattering (HFS),preferably contains oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 5 at. % to 30 at. %, 20 at. % to 55 at. %,25 at. % to 35 at. %, and 10 at. % to 30 at. %, respectively.

“Silicon oxynitride” contains more oxygen than nitrogen and in the casewhere measurements are performed using RBS and HFS, preferably containsoxygen, nitrogen, silicon, and hydrogen at 50 at. % to 70 at. %, 0.5 at.% to 15 at. %, 25 at. % to 35 at. %, and 0.1 at. % to 10 at. %,respectively.

Note that percentages of nitrogen, oxygen, silicon, and hydrogen fallwithin the ranges given above, where the total number of atoms containedin the silicon oxynitride or the silicon nitride oxide is defined as 100at. %.

The semiconductor film 110 may be a single layer or a stacked-layer ofplural layers as long as it is a semiconductor film(s). As thesemiconductor film 110, for example, an oxide semiconductor film or asilicon film can be used.

When the semiconductor film 110 is an oxide semiconductor film, afour-component metal oxide such as an In—Sn—Ga—Zn-based oxidesemiconductor; a three-component metal oxide such as an In—Ga—Zn-basedoxide semiconductor, an In—Sn—Zn-based oxide semiconductor, anIn—Al—Zn-based oxide semiconductor, a Sn—Ga—Zn-based oxidesemiconductor, an Al—Ga—Zn-based oxide semiconductor, or aSn—Al—Zn-based oxide semiconductor; a two-component metal oxide such asan In—Zn-based oxide semiconductor, a Sn—Zn-based oxide semiconductor,an Al—Zn-based oxide semiconductor, a Zn—Mg-based oxide semiconductor, aSn—Mg-based oxide semiconductor, an In—Ga-based oxide semiconductor, oran In—Mg-based oxide semiconductor; an In-based oxide semiconductor, aSn-based oxide semiconductor, or a Zn-based oxide semiconductor; or thelike can be used. Such an oxide semiconductor film can contain SiO₂. Forexample, an In—Ga—Zn-based oxide semiconductor film may be an oxidesemiconductor film having In, Ga, or Zn, regardless of its stoichiometryratio. Further, the In—Ga—Zn-based oxide semiconductor film may containan element other than In, Ga, and Zn.

When an oxide semiconductor film is used for the semiconductor film 110,an oxide semiconductor film represented by the chemical formula,InMO₃(ZnO)_(m) (m>0) can also be used for example. Here, M representsone or more metal elements selected from the group of Ga, Al, Mn, andCo. For example, M can be Ga, Ga and Al, Ga and Mn, or Ga and Co. Inaddition, the oxide semiconductor film may include SiO₂.

A target used for the formation of the oxide semiconductor film by asputtering method was, for example, an oxide target containing In₂O₃,Ga₂O₃, and ZnO at a composition ratio of 1:1:1 [molar ratio]. However,without being limited to the material and the composition ratio, forexample, an oxide semiconductor target containing In₂O₃, Ga₂O₃, and ZnOat a composition ratio of 1:1:2 [molar ratio] may be used.

Here, in a case where the semiconductor film 110 is an oxidesemiconductor film formed by a sputtering method, the semiconductor film110 can be formed by the sputtering method under a rare gas (for exampleAr) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a raregas and oxygen.

The filling rate of the oxide target is 90% to 100%, preferably 95% to99.9%. With use of the target having a high filling factor, the oxidesemiconductor film to be formed can be a dense film.

Alternatively, a silicon film may be formed as the semiconductor film110. The silicon film may be an amorphous silicon film. Alternatively,the semiconductor film 110 may be a stacked-layer silicon film in whicha silicon film with low carrier mobility is stacked over a silicon filmwith high carrier mobility.

An example of the silicon film with high carrier mobility is acrystalline silicon film, and an example of the crystalline silicon ismicrocrystalline silicon. Note that microcrystalline silicon is asemiconductor having an intermediate structure between an amorphousstructure and a crystalline structure (including single crystal andpolycrystal). Microcrystalline silicon is silicon having a third statethat is stable in terms of free energy and is crystalline silicon havingshort-range order and lattice distortion, in which column-like orneedle-like crystal grains having a diameter of 2 nm to 200 nm,preferably 10 nm to 80 nm, more preferably 20 nm to 50 nm grow in adirection normal to the substrate surface. Thus, there is a case wheregrain boundaries are formed at the interface of the columnar orneedle-like crystal grains. It is to be noted that the crystal grainsize means the maximum diameter of a crystal grain in a plane parallelto the substrate surface. Further, the crystal grain includes anamorphous silicon region and a crystallite which is a minute crystalthat can be regarded as a single crystal. It is to be noted that thecrystal grains may include a twin crystal in some cases.

Microcrystalline silicon, which is one of microcrystallinesemiconductors, has a peak of Raman spectrum which is shifted to a lowerwave number side than 520 cm⁻¹ that features single crystal silicon.More specifically, the Raman spectrum peak of the microcrystallinesilicon lies between the wave number of 520 cm⁻¹ that features singlecrystalline silicon and the wave number of 480 cm⁻¹ that featuresamorphous silicon. The microcrystalline silicon may contain hydrogen orhalogen of at least 1 at. % to terminate dangling bonds. Furthermore,the microcrystalline silicon contains a rare gas element such as He, Ar,Kr, or Ne to further promote lattice distortion, so that stability isincreased and favorable microcrystalline silicon can be obtained.

The silicon film with low carrier mobility may be an amorphous siliconfilm, preferably a silicon film which includes amorphous silicon andgrains of micro-crystalline silicon and has lower energy at an Urbachedge measured by a constant photocurrent method (CPM) orphotoluminescence spectroscopy and a smaller amount of defect-inducedabsorption spectrum, as compared with conventional amorphous siliconfilms. This silicon film is a well-ordered silicon film which has fewerdefects and has a level with a steeper tail slope at the band edge(mobility edge) of the valence band, as compared with conventionalamorphous silicon films.

The silicon film with low carrier mobility may contain a halogen elementor nitrogen. In the case of containing nitrogen, the silicon film withlow carrier mobility may contain nitrogen as a NH group or a NH₂ group.

Note that an interface region between the silicon film with high carriermobility and the silicon film with low carrier mobility includesmicrocrystalline semiconductor regions and amorphous semiconductorregions among the microcrystalline semiconductor regions. Specifically,the interface region includes a microcrystalline semiconductor regionwhich grows in a conical or pyramidal shape from the silicon film withhigh carrier mobility, and a region including amorphous semiconductorsimilar to the silicon film with low carrier mobility.

When the silicon film with low carrier mobility is provided between thesource and drain electrodes and the silicon film with high carriermobility, off-state current of the transistor can be reduced. Further,since the interface region has the microcrystalline silicon region whichextends in a conical or pyramidal shape, resistance in the verticaldirection (the direction in which the film grows) can be lowered andon-state current of the transistor can be increased. In other words, ascompared to the case of using conventional amorphous silicon, theoff-state current can be sufficiently reduced and reduction in on-statecurrent can be inhibited; thus, switching characteristics of thetransistor can be enhanced.

Note that the microcrystalline silicon region grows from a surface ofthe silicon film with high carrier mobility in the film thicknessdirection. When the flow rate of hydrogen to that of a deposition gas(e.g., silane) in a source gas is low (that is, the dilution ratio islow) or when the concentration of a source gas containing nitrogen ishigh, crystal growth in the microcrystalline silicon region issuppressed, and thus, crystal grains come to have a conical or pyramidalshape, and a large portion of the deposited silicon is amorphous.

As the second etching mask 112, for example, a resist mask may beformed; however, there is no particular limitation on the second etchingmask 112, and any material that can be used as a mask in an etching stepcan be used.

Then, with use of the second etching mask 112, the semiconductor film110 is processed so that the semiconductor layer 114 is formed (FIG.1D).

Then, the second etching mask 112 is removed, and a transparentconductive film 116 is formed to cover the semiconductor layer 114 overthe gate insulating layer 108 (FIG. 2A).

The transparent conductive film 116 can be formed using a conductivecomposition including a conductive high molecule (also referred to as aconductive polymer) having a light-transmitting property. It ispreferred that the transparent conductive film 116 formed using theconductive composition has a sheet resistance of 10000 W/square or lessand a light transmittance of 70% or higher at a wavelength of 550 nm.Further, the resistivity of the conductive high molecule included in theconductive composition is preferably 0.1 Ωcm or lower.

Next, a third etching mask 118 is formed over the transparent conductivefilm 116 (FIG. 2B).

As the third etching mask 118, for example, a resist mask may be formed;however, there is no particular limitation on the third etching mask118, and any material that can be used as a mask in an etching step canbe used.

Next, the transparent conductive film 116 is processed using the thirdetching mask 118 so that an electrode 120 a also serving as a signalline and an electrode 120 b also serving as a pixel electrode are formed(FIG. 2C).

After that, the third etching mask 118 is removed (FIG. 2D).

After that, preferably, an upper portion of the semiconductor layer 114is etched so that residue or the like left between the electrode 120 aalso serving as a signal line and the electrode 120 b also serving as apixel electrode is removed and thereby the electrodes are completelyinsulated.

In the above-described manner, the pixel transistor can be fabricated. Atop view of the pixel transistor is illustrated in FIG. 3. In the pixeltransistor in this embodiment, a contact resistance is not generatedbetween the drain electrode (or the source electrode) and the pixelelectrode, because the drain electrode (or the source electrode) alsoserves as the pixel electrode.

In addition, in accordance with the fabrication method of a pixeltransistor in this embodiment, the number of masks can be reduced ascompared with that in a conventional method. Specifically, a pixeltransistor can be fabricated with use of three masks.

Note that over the electrode 120 a also serving as a signal line, a lowresistant conductive layer 122 is preferably provided (FIG. 4A). The lowresistant conductive layer 122 formed over the electrode 120 a alsoserving as a signal line is preferably formed without increasing thenumber of masks. In order to form the low resistant conductive layer 122over the electrode 120 a also serving as a signal line withoutincreasing the number of masks, a metal layer is preferably formed by anelectroplating method.

In other words, the substrate 100 provided with the pixel transistorillustrated in FIG. 2D and an anode formed of a metal to be used forplating or an insoluble metal are immersed in an electrolyte solutionincluding ions of the metal to be used for plating, and thus a potentialdifference is generated between the anode and the electrode 120 a alsoserving as a signal line so that cations are reduced on the surface ofthe electrode 120 a also serving as a signal line and thus a metal layeris formed.

Here, preferred examples of the metal to be used for plating include,but not limited to, silver, gold, and platinum, which are metals havinglow electric resistances and low ionization tendencies.

After that, preferably, an upper portion of the semiconductor layer 114is etched so that residue or the like left between the electrode 120 aalso serving as a signal line and the electrode 120 b also serving as apixel electrode is removed to make complete insulation, and then thesemiconductor layer 124 is formed (FIG. 4B).

FIG. 5 illustrates a top view of the pixel transistor fabricated in thismanner. In the pixel transistor illustrated in FIG. 5, wiring resistanceis lowered due to the low resistant conductive layer and no contactresistance is generated between the drain electrode (or the sourceelectrode) and the pixel electrode. According to this embodiment, such adisplay device can be manufactured with the small number of masks.

Embodiment 2

The first etching mask 104 and the second etching mask 112 in Embodiment1 can be formed using the same photomask. Therefore, the number ofphotomasks can be smaller than that in Embodiment 1. In this embodiment,a manufacturing method of a display device with the smaller number ofphotomasks than that in Embodiment 1 will be described with reference toFIGS. 6A and 6B, FIGS. 7A and 7B, FIGS. 8A and 8B, FIGS. 9A and 9B, andFIGS. 10A and 10B.

Note that FIG. 6A, FIG. 7A, FIG. 8A, FIG. 9A, and FIG. 10A are topviews, while FIG. 6B, FIG. 7B, FIG. 8B, FIG. 9B, FIG. 10B arecross-sectional views taken along the lines A-B in FIG. 6A, FIG. 7A,FIG. 8A, FIG. 9A, and FIG. 10A.

In a manner similar to that in Embodiment 1, the first conductive film102 is formed over the substrate 100, and the first etching mask 104 isformed over the first conductive film 102 (FIGS. 6A and 6B).

Next, with use of the first etching mask 104, the first conductive film102 is processed so that the gate electrode 106 is formed, the firstetching mask 104 is removed, the gate insulating layer 108 is formed tocover the gate electrode 106, the semiconductor film 110 is formed overthe gate insulating layer 108, and a second etching mask precursor 111is formed over the semiconductor film 110 (FIGS. 7A and 7B).

Here, the second etching mask precursor 111 is formed with use of thesame photomask as the first etching mask 104.

Next, the second etching mask precursor 111 is scaled down so that thesecond etching mask 112 is formed (FIGS. 8A and 8B). For scaling downthe second etching mask precursor 111, an aching process may beperformed for example.

Here, the second etching mask 112 is formed to be island-shaped. Thatis, the second etching mask precursor 111 is scaled down so that thesecond etching mask 112 can have an island shape. At this time, in thecase where the width of an ashed portion of the second etching maskprecursor 111 is “a” and the width of a scan line formed of the gateelectrode 106 is “b”, it is only necessary that the b may be smallerthan 2 a.

That is, in this embodiment, the relation, b<2 a should be fulfilled.However, when the width of the scan line is too narrow, defectiveformation might occur, so that ample current is difficult to flow. Forthat reason, the width b of the scan line is preferably larger than thewidth a. Accordingly, it is much preferable that the relation a≦b≦2 a isfulfilled.

Next, the semiconductor film 110 is processed with use of the secondetching mask 112 so that the semiconductor layer 114 is formed, thesecond etching mask 112 is removed, the transparent conductive film 116is formed to cover the semiconductor layer 114 over the gate insulatinglayer 108, and the third etching mask 118 is formed over the transparentconductive film 116 (FIGS. 9A and 9B).

With use of the third etching mask 118, the transparent conductive film116 is processed so that the electrode 120 a also serving as a signalline and the electrode 120 b also serving as a pixel electrode areformed, and then the third etching mask 118 is removed (FIGS. 10A and10B).

After that, preferably, the low resistant conductive layer 122 is formedas in Embodiment 1.

As described in this embodiment, the pixel transistor can be fabricatedwith two photomasks.

Embodiment 3

When the pixel transistor in which the low resistant conductive layer isformed over the electrode also serving as a signal line is formed, asthe preferred embodiments described in Embodiment 1 and Embodiment 2, itis difficult to make the channel lengths within the substrate uniform.Therefore, in this embodiment, an embodiment in which a sidewallinsulating layer is provided for the electrode also serving as a signalline and the electrode also serving as a pixel electrode will bedescribed. When a sidewall insulating layer is provided for theelectrode also serving as a signal line and the electrode also servingas a pixel electrode, the distance (channel length L) between theelectrode also serving as a signal line and the electrode also servingas a pixel electrode can be determined at the time of processing of thetransparent conductive film, so that the channel lengths L can beprevented from varying from point to point within the substrate.Accordingly, display unevenness (mura) of a display device can beprevented.

First, in a manner similar to that in Embodiment 1, elements up to andincluding the electrode 120 a also serving as a signal line and theelectrode 120 b also serving as a pixel electrode are formed.

Then, after the third etching mask 118 is removed, a sidewall insulatingfilm 126 is formed over the gate insulating layer 108, the semiconductorlayer 114, the electrode 120 a also serving as a signal line and theelectrode 120 b also serving as a pixel electrode (FIG. 11A).

The sidewall insulating film 126 can be formed of an insulating film(for example, silicon nitride, silicon nitride oxide, siliconoxynitride, silicon oxide, or the like) by, for example, a sputteringmethod or a CVD method (including a plasma CVD method, a thermal CVDmethod, and the like). Note that the sidewall insulating film 126 may beformed to have a single layer structure or a stacked structure includinga plurality of layers.

Next, a sidewall insulating layer 128 is formed by performing etch-backtreatment so as to expose the electrode 120 a also serving as a signalline and the electrode 120 b also serving as a pixel electrode (FIG.11B).

Next, the low resistant conductive layer 122 is formed over theelectrode 120 a also serving as a signal line by an electroplatingmethod (FIG. 11C).

As described above, a sidewall insulating layer can be formed on theelectrode 120 a also serving as a signal line and the electrode 120 balso serving as a pixel electrode. In this manner, the provision of thesidewall insulating layer for the electrode 120 a also serving as asignal line and the electrode 120 b also serving as a pixel electrodecan determine the distance (channel length L) between the electrode 120a also serving as a signal line and the electrode 120 b also serving asa pixel electrode at the time of processing the transparent conductivefilm 116, irrespective of formation of the low resistant conductivelayer 122 so that the channel lengths L can be prevented from varyingfrom point to point within the substrate. Therefore, display unevenness(mura) of the display device can be prevented.

After that, preferably, an upper portion of the semiconductor layer 114is etched so that residue or the like left between the electrode 120 aalso serving as a signal line and the electrode 120 b also serving as apixel electrode is removed to make complete insulation and then thesemiconductor layer 124 is formed (FIG. 11D).

Embodiment 4

In Embodiment 1, as the method of forming the low resistant conductivelayer 122 without increasing the number of masks, an electroplatingmethod is exemplified. However, the method is not a limiting example,and another method can be employed to faun the low resistant conductivelayer 122. In this embodiment, a lift-off process is used as the method.

Note that the term “lift-off” process means such a process that in orderto make a desired pattern, a protective mask with the desired pattern isformed, a film to be provided with the desired pattern is formed, andthen the protective mask is removed, so that the desired pattern isformed in a portion which have not been provided with the protectivemask.

First, in a manner similar to that in Embodiment 1, elements up to andincluding the electrode 120 a also serving as a signal line and theelectrode 120 b also serving as a pixel electrode are formed. Then, aprotective mask 130 is formed in a portion excluding the portion wherethe low resistant conductive layer 122 is formed (FIG. 12A). At thistime, the protective mask 130 may be formed of a resist material, forexample.

Next, a low resistant conductive film 132 for formation of the lowresistant conductive layer 122 is formed (FIG. 12B). At this time, thelow resistant conductive film 132 is formed entirely and thus is formedalso over the protective mask 130.

Next, the lift-off process is performed to remove the protective mask130. By the removal of the protective mask 130, the low resistantconductive film 132 over the protective mask 130 is also removed, sothat the low resistant conductive layer 122 in a desired region isformed (FIG. 12C).

After that, preferably, an upper portion of the semiconductor layer 114is etched to remove the residue or the like left between the electrode120 a also serving as a signal line and the electrode 120 b serving asthe pixel electrode to make complete insulation, and then thesemiconductor layer 124 is formed (FIG. 12D).

As described above, the low resistant conductive layer 122 can be formedwithout increasing the number of masks such as photomasks and etchingmasks and without performing an electroplating method. It is noted thatby the lift-off process, a minute pattern of about 2 μm or less isdifficult to be formed. However, for example, the width of a signal lineof a large-screen liquid crystal display device using amorphoussemiconductor is about 3 μm or more, and as described in thisembodiment, the low resistant conductive layer 122 is formed only in theportion of the signal line, not necessarily in the electrode portion.Therefore, the difficulty in miniaturization of the lift-off processdoes not matter.

The lift-off process described in this embodiment can be applied to thefabrication methods of the pixel transistors described in Embodiment 2and Embodiment 3.

Embodiment 5

In Embodiment 1 and Embodiment 4, the lift-off process and theelectroplating method are exemplified as the method of forming the lowresistant conductive layer 122 without increasing the number of maskssuch as photomasks and etching masks. However, the methods are notlimiting examples, and another method can be employed to form the lowresistant conductive layer 122. In this embodiment, a droplet-dischargemethod is used as the method.

Note that the “droplet-discharge method” means such a method that adroplet of a composition for forming a desired layer is selectivelydropped or injected so that the desired pattern is formed. An inkjetmethod is included in the category of the droplet-discharge method.

First, in a manner similar to that in Embodiment 1, elements up to andincluding the electrode 120 a also serving as a signal line and theelectrode 120 b also serving as a pixel electrode are formed. Then, alow resistant conductive film 134 is formed, a fourth etching mask 136is formed over the low resistant conductive film 134 by adroplet-discharge method (FIG. 13A). At this time, the fourth etchingmask 136 can be formed of a resist material for example.

Next, with use of the fourth etching mask 136, the low resistantconductive film 134 is etched so that the low resistant conductive layer122 is formed (FIG. 13B). Then, the fourth etching mask 136 is removed(FIG. 13C).

After that, preferably, an upper portion of the semiconductor layer 114is etched to remove the residue or the like left between the electrode120 a also serving as a signal line and the electrode 120 b serving asthe pixel electrode to make complete insulation, and then thesemiconductor layer 124 is formed (FIG. 13D).

As described above, the low resistant conductive layer 122 can be formedwithout increasing the number of photomasks and without performing anelectroplating method or a lift-off process. It is noted that by thedroplet-discharge method, a minute pattern of about 2 μm or less isdifficult to be formed. However, for example, the width of a signal lineof a large-screen liquid crystal display device using amorphoussemiconductor is about 3 μm or more, and as described in thisembodiment, the low resistant conductive layer 122 is formed only in theportion of the signal line, not necessarily in the electrode portion.Therefore, the difficulty in miniaturization of the droplet-dischargemethod does not matter.

Note that one embodiment of the present invention, described in thisembodiment, is not limited to the description made above, and the lowresistant conductive layer 122 itself may be formed by adroplet-discharge method.

Note that the droplet-discharge method described in this embodiment maybe applied to the fabrication methods of the pixel transistors describedin Embodiment 2 and Embodiment 3.

The droplet-discharge method described in this embodiment can becombined with the lift-off process described in Embodiment 4. In otherwords, the protective mask 130 in Embodiment 4 can be formed by adroplet-discharge method.

Embodiment 6

As described in Embodiment 1, when the low resistant conductive layer122 is formed over the electrode 120 a also serving as a signal line byan electroplating method, the potentials of the electrodes 120 a alsoserving as a signal line are preferably equal for electroplating so thatthe low resistant conductive layer 122 can be formed uniformly over thesubstrate. In this embodiment described is a production system of adisplay device in which electroplating is conducted with the equalpotentials of the electrodes 120 a also serving as a signal line.

First, in a manner similar to that in Embodiment 1, elements up to andincluding the electrode 120 a also serving as a signal line and theelectrode 120 b also serving as a pixel electrode are formed.

FIG. 14A is a schematic diagram of the substrate 100 where elements upto and including the electrode 120 a also serving as a signal line andthe electrode 120 b also serving as a pixel electrode are formed asdescribed in Embodiment 1. Over the substrate 100, a plurality of pixels150 are formed in matrix. In addition, each input terminal supplies asignal or a power potential to the pixels 150.

The plurality of pixels 150 are each connected to a signal line 152 anda scan line 154, and a signal line side input terminal 156 and a scanline side input terminal 158 which supply signals or power potentialsare formed in the outer periphery of the substrate 100.

An upper arm 170 and a lower arm 172 used to transport the substrate 100are disposed so as to hold the signal line side input terminal 156 ofthe substrate 100 in a region 160. The upper arm 170 and the lower arm172 may hold one signal line side input terminal 156 over the substrate100, or hold both of the signal line side input terminals 156 providedon the opposite sides.

FIG. 14B illustrates where the upper arm 170, the lower arm 172, and thesubstrate 100 in FIG. 14A are positioned. As illustrated in FIG. 14B,the upper arm 170 and the lower arm 172 hold the substrate 100. Aconductive portion 174 and a conductive portion 176 are provided for theupper arm 170 and the lower arm 172 respectively, and the conductiveportion 174 or the conductive portion 176 is connected to the signalline side input terminal 156.

The substrate 100 held by the upper arm 170 and the lower arm 172 isimmersed in an electrolyte solution containing ions of a metal to beused for plating together with the anode formed of the metal to be usedfor plating or an insoluble metal. When the substrate 100 is immersed inthe electrolyte solution, a potential difference is made to be producedbetween the anode and the conductive portion 174 or the conductiveportion 176, and thereby cations are reduced on the surface of theelectrode 120 a also serving as a signal line so that the low resistantconductive layer is formed.

In the above-described manner, an active matrix substrate of a displaydevice including the pixel transistor described in Embodiment 1 can bemanufactured. Note that with use of a production system of thisembodiment, an active matrix substrate of a display device including thepixel transistor described in Embodiment 2 can be manufactured as well.

Embodiment 7

Electronic paper can be given as an example of a semiconductor device towhich the pixel transistor fabricated in the above-described manner isapplied. Electronic paper can be used for electronic devices of avariety of fields for displaying information. For example, electronicpaper can be applied to e-book readers (e-books), posters, digitalsignage, public information displays (PID), advertisements in vehiclessuch as trains, displays of various cards such as credit cards, and thelike. An example of an electronic device is illustrated in FIG. 15.

FIG. 15 illustrates an example of an e-book reader. For example, ane-book reader 200 includes two housings 202 and 204. The housing 202 andthe housing 204 are unified with a hinge 214 so that the e-book reader200 can be opened and closed with the hinge 214 as an axis. With such astructure, the e-book reader 200 can be handled like a paper book.

A display portion 206 and a photoelectric conversion device 208 areincorporated in the housing 202. A display portion 210 and aphotoelectric conversion device 212 are incorporated in the housing 204.The display portion 206 and the display portion 210 may be configured todisplay one image or different images. When different images aredisplayed, for example, text can be displayed on the right displayportion (the display portion 206 in FIG. 15) and images can be displayedon the left display portion (the display portion 210 in FIG. 15).

FIG. 15 illustrates the example in which the housing 202 is providedwith an operation portion and the like. For example, the housing 202 isprovided with a power switch 216, operation keys 218, a speaker 220, andthe like. Pages can be turned with the operation keys 218. Note that akeyboard, a pointing device, or the like may also be provided on thesurface of the housing, on which the display portion is provided.Further, an external connection terminal (an earphone terminal, a USBterminal, a terminal connectable to an AC adapter or a variety of cablessuch as a USB cable, or the like), a recording medium insertion portion,and the like may be provided on the back surface or the side surface ofthe housing. Furthermore, the e-book reader 200 may have a function ofan electronic dictionary.

The e-book reader 200 may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

Embodiment 8

Further, as semiconductor devices to which the pixel transistor formedas described above is applied, a variety of electronic devices(including an amusement machine) can be given in addition to electronicpaper. Examples of electronic devices are television devices (alsoreferred to as televisions or television receivers), monitors ofcomputers and the like, cameras such as digital cameras or digital videocameras, digital photo frames, mobile phones (also referred to ascellular phones or mobile phone devices), portable game consoles,personal digital assistants, audio reproducing devices, large-sized gamemachines such as pachinko machines, and the like.

FIG. 16A illustrates an example of a television device. In a televisionset 222, a display portion 226 is incorporated in a housing 224. Thedisplay portion 226 can display images. Further, the housing 224 issupported by a stand 228 here.

The television device 222 can be operated by an operation switch of thehousing 224 or a separate remote controller 234. Channels and volume canbe controlled by an operation key 232 of the remote controller 234, sothat an image displayed on the display portion 226 can be manipulated.Further, the remote controller 234 may be provided with a displayportion 230 which displays data output from the remote controller 234.

Note that the television set 222 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set 222 isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 16B illustrates an example of a digital photo frame. For example,in a digital photo frame 236, a display portion 240 is incorporated in ahousing 238. The display portion 240 can display various kinds ofimages. For example, the display portion 240 can display data of animage shot by a digital camera or the like, and thereby functioning as anormal photo frame.

Note that the digital photo frame 236 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalconnectable to various cables such as a USB cable, or the like), arecording medium insertion portion, and the like. Although thesecomponents may be provided on the surface on which the display portionis provided, it is preferable to provide them on the side surface or theback surface for the design of the digital photo frame 236. For example,a memory storing data of an image shot by a digital camera is insertedinto the recording medium insertion portion of the digital photo frame,whereby the image data can be transferred and then displayed on thedisplay portion 240.

The digital photo frame 236 may have a configuration capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, desired image data can be downloaded and be displayed.

FIG. 17 is a perspective view illustrating an example of a portablecomputer.

In a portable computer in FIG. 17, a top housing 242 having a displayportion 246 and a bottom housing 244 having a keyboard 248 can overlapwith each other by closing the portable computer with a hinge unit whichconnects the top housing 242 and the bottom housing 244. The portablecomputer is convenient for carrying around. Moreover, in the case ofusing the keyboard for input, the portable computer is open with thehinge unit so that a user can input looking at the display portion 246.

The bottom housing 244 includes a pointing device 252 with which inputcan be performed, in addition to the keyboard 248. Further, when thedisplay portion 246 is used as a touch input panel, input can beperformed by touching part of the display portion. The bottom housing244 includes an arithmetic function portion such as a CPU or hard disk.In addition, the bottom housing 244 includes an external connection port250 into which another device such as a communication cable conformableto communication standards of a USB is inserted.

The top housing 242 further includes a display portion 254 which can bestored in the top housing 242 by being slid therein. With the displayportion 254, a large display screen can be realized. In addition, theuser can adjust the orientation of a screen of the storable displayportion 254. When the storable display portion 254 is used as a touchinput panel, input can be performed by touching part of the storabledisplay portion.

The display portion 246 or the storable display portion 254 is formedusing an image display device such as a liquid crystal display panel ora light-emitting display panel including an organic light-emittingelement, an inorganic light-emitting element, or the like.

In addition, the portable computer in FIG. 17 can be provided with areceiver and the like and can receive television broadcasting to displayimages on the display portion. The user can watch televisionbroadcasting when the whole screen of the display portion 254 is exposedby sliding the display portion 254 while the hinge unit which connectsthe top housing 242 and the bottom housing 244 is kept closed. In thiscase, the hinge unit is not opened and display is not performed on thedisplay portion 246, and start up of only a circuit for displayingtelevision broadcasting is performed. Therefore, power consumption canbe minimized, which is advantageous for the portable computer whosebattery capacity is limited.

This application is based on Japanese Patent Application serial No.2010-267505 filed with the Japan Patent Office on Nov. 30, 2010, theentire contents of which are hereby incorporated by reference.

1. A display device comprising: a gate electrode; a gate insulatinglayer over the gate electrode; a semiconductor layer over the gateinsulating layer; and a source electrode and a drain electrode separatedfrom each other and in contact with the semiconductor layer, wherein oneof the source electrode and the drain electrode serves as a pixelelectrode.
 2. The display device according to claim 1, wherein at leastone of the source electrode and the drain electrode is transparent.
 3. Adisplay device comprising: a gate electrode; a gate insulating layerover the gate electrode; a semiconductor layer over the gate insulatinglayer; a source electrode and a drain electrode separated from eachother and in contact with the semiconductor layer; and a low resistantconductive layer on one of the source electrode and the drain electrode,wherein the one of the source electrode and the drain electrode servesas a signal line, and wherein the other of the source electrode and thedrain electrode serves as a pixel electrode.
 4. The display deviceaccording to claim 3, wherein at least one of the source electrode andthe drain electrode is transparent.
 5. A method for manufacturing adisplay device, comprising the steps of: forming a first conductive filmover a substrate; forming a first etching mask over the first conductivefilm; processing the first conductive film with use of the first etchingmask, whereby a gate electrode is formed; removing the first etchingmask; forming a gate insulating layer over the gate electrode; forming asemiconductor film over the gate insulating layer; forming a secondetching mask over the semiconductor film; processing the semiconductorfilm with use of the second etching mask, whereby a semiconductor layeris formed; removing the second etching mask; forming a second conductivefilm over the gate insulating layer to cover the semiconductor layer;forming a third etching mask over the second conductive film; processingthe second conductive film with use of the third etching mask, wherebyan electrode serving as a signal line and an electrode serving as apixel electrode are formed; and removing the third etching mask.
 6. Themethod for manufacturing a display device, according to claim 5, whereinthe second conductive film is transparent.
 7. The method formanufacturing a display device, according to claim 5, further comprisingthe steps of: after the step of removing the third etching mask, forminga sidewall insulating film over the gate insulating layer, thesemiconductor layer, the electrode serving as the signal line, and theelectrode serving as the pixel electrode; etching the sidewallinsulating film back to expose the electrode serving as the signal lineand the electrode serving as the pixel electrode, whereby a sidewallinsulating layer is formed; forming a low resistant conductive layer onthe electrode serving as the signal line by an electroplating method. 8.The method for manufacturing a display device, according to claim 5,further comprising the step of: etching the electrode serving as thesignal line and the electrode serving as the pixel electrode, afterremoving the third etching mask.
 9. The method for manufacturing adisplay device, according to claim 7, further comprising the step of:etching the electrode also serving as the signal line and the electrodealso serving as the pixel electrode, after forming the low resistantconductive layer.
 10. A method for manufacturing a display device,comprising the steps of: forming a first conductive film over asubstrate; forming a first etching mask over the first conductive film;processing the first conductive film with use of the first etching mask,whereby a gate electrode is aimed; removing the first etching mask;forming a gate insulating layer over the gate electrode; forming asemiconductor film over the gate insulating layer; forming a secondetching mask over the semiconductor film; processing the semiconductorfilm with use of the second etching mask, whereby a semiconductor layeris formed; removing the second etching mask; forming a second conductivefilm over the gate insulating layer to cover the semiconductor layer;forming a third etching mask over the second conductive film; processingthe second conductive film with use of the third etching mask, wherebyan electrode serving as a signal line and an electrode serving as apixel electrode are formed; removing the third etching mask; and forminga low resistant conductive film on the electrode serving as the signalline.
 11. The method for manufacturing the display device, according toclaim 10, wherein the second conductive film is transparent.
 12. Themethod for manufacturing the display device, according to claim 10,wherein the low resistant conductive film is formed by an electroplatingmethod.
 13. The method for manufacturing a display device, according toclaim 10, further comprising the steps of: after the step of removingthe third etching mask, forming a sidewall insulating film over the gateinsulating layer, the semiconductor layer, the electrode serving as thesignal line, and the electrode serving as the pixel electrode; etchingthe sidewall insulating film back to expose the electrode serving as thesignal line and the electrode serving as the pixel electrode, whereby asidewall insulating layer is formed.
 14. The method for manufacturing adisplay device, according to claim 10, further comprising the steps of:after the step of removing the third etching mask, forming a protectivemask by an inkjet method in a portion excluding a portion where a lowresistant conductive layer is to be formed, and lifting off theprotective mask after forming the low resistant conductive film, wherebythe low resistant conductive layer is formed.
 15. The method formanufacturing a display device, according to claim 10, furthercomprising the steps of: forming a fourth etching mask by an inkjetmethod in a region where a low resistant conductive layer is to beformed over the low resistant conductive film; processing the lowresistant conductive film with use of the fourth etching mask, wherebythe low resistant conductive layer is formed; and removing the fourthetching mask.
 16. The method for manufacturing a display device,according to claim 14, further comprising the step of: etching theelectrode also serving as the signal line and the electrode also servingas the pixel electrode, after lifting off the protective mask.
 17. Themethod for manufacturing a display device, according to claim 15,further comprising the step of: etching the electrode also serving asthe signal line and the electrode also serving as the pixel electrode,after removing the fourth etching mask.